: The sellers of most expansion tanks have a chart that gives guidelines for the size of their tank that's needed for a given water heater.

: I did the math once, and came up with around 1.7 gallons expansion for a 50 gallon tank, assuming a reasonable temperature range for reheating, although I don't remember at the moment what specific temperatures I assumed.

: Do you think you need one? One way to check is to connect a pressure gage to an outside faucet. After you've finished your morning showers, do not open another faucet or flush a toilet for about 1/2 to 1 hour. If the pressure gage rises significantly, you may need an expansion tank. If backflow into the city water main is blocked by a check valve or other device, the pressure will probably rise to 150 psi, where it will then be relieved at the hot water heater relief valve.

: - John

: : I have read several messages discussing the use of a thermal expansion tank on domestic hot water systems supplied from a pressure regulator or a back flow preventer. Is there a way of determining the volumetric size of the expansion tank? Is it based on the volume of the hot water tank (mine is 50 gal), or the hysteresis of the thermostat controlling the temperature? My guess is that both are involved, but perhaps there is a simple rule of thumb that is used. Thanks for the help.

I definitely need an expansion tank. I have a pressure regulator on my system because the hot water tank relief valve is only rated for 100 psi and line pressure generally exceeds that. I asked the manufacturer about changing the valve to 150 psi, like all my neighbours have, but they told me that would void the warranty. My regulator is set for 60 psi, but whenever the tank heats up the relief valve discharges water. The volume of water discharged is only about one pint. I have also located a manufacturer on the web and they have a chart, as you suggested.